AB0129 MYOSITIS-SPECIFIC AUTOANTIBODIES IN CLINICAL PRACTICE: IMPROVING THE PERFORMANCES OF THE IMMUNODOT
Authors: E. Bories, F. Fortenfant, G. Pugnet, Y. Renaudineau, C. Bost
Journal: Annals of the Rheumatic Diseases (2022)

Background Idiopathic inflammatory myopathies (IIM) or myositis are a group of rare autoimmune diseases that combine muscle weakness and multi-visceral damage. The discovery of IIM-specific autoantibodies (aAbs) and their associations with clinical phenotypes has improved diagnostic and classification criteria. Faced with the large number of these aAbs, multiplexed techniques have emerged. Among them, the immunodot is simple, rapid, and inexpensive, but has been several times criticized for its lack of specificity. Objectives Our objective was to evaluate the current interpretation criteria of the D-Tek immunodot and to propose new interpretation rules based on clinical criteria in order to improve its reliability. Methods Were included in this retrospective study patients tested positive result on the semi-quantitative myositis/synthetase immunodots at manufacturer threshold (≥ 5 UA), for at least one of the 15 aAbs: anti-SRP, anti-NXP2, anti-TIF1, anti-SAE (1 and 2), anti-Mi2, anti-MDA5, anti-Jo1, anti-PL7, anti-PL12, anti-EJ, anti-OJ, anti-KS, anti-ZO, anti-HA. Specificity of the immunodots was further evaluated using 60 healthy and anonymous subjects (French blood bank, Toulouse, France). The clinical diagnosis and sub-classification retained by the clinician in charge of the patient was used as a reference for attribution to the myositis/non-myositis group and subgroups. For the myositis group, 7 subgroups were considered: immune-mediated necrotizing myopathy (n=4); dermatomyositis (n=66); anti-synthetase syndrome (n=36); inclusion body myositis (n=1); overlap myositis with another connective tissue disease (n=7); polymyositis (n=8); and unclassified myositis (n=6). For the non-myositis group, patients were subdivided in 4 subgroups: autoimmune or inflammatory diseases (n=72); isolated and diffuse interstitial lung disease (n=26) not associated with other myositis criteria; other non-inflammatory myopathies (n=8) including genetic, metabolic, and toxic myopathies; and other diseases (n=36). The immunodot interpretation thresholds were evaluated both in relation to the manufacturer’s threshold, and by considering the phenotypes and clinical diagnoses using a ROC method (Youden’s index). Results Among 270 patients included between 01/07/2016 and 30/06/2020, 128 (47%) were classified as myositis (median age 58 years, 60% women, 52% DM and 28% AS) and 142 (53%) in non-myositis. Among the 15 aAbs analyzed, none were detected in the healthy control group but they were represented in both myositis and non-myositis group. Among them only 2 (anti-Jo1, anti-Mi2) predominate in the myositis group, and 1 (anti-TIF1) in the non-myositis group (Fisher’s test). As quantitative values were found different for 5 aAbs (Mann Whitney test), a clinical threshold was calculated to discriminate myositis from non-myositis groups (ROC curve) allowing to determine an odds ratio (OR). Accordingly, 4/15 (%) aAbs were found associated with myositis: anti-SRP (at 28UA: OR=3.24 95% CI [1.01-10.46], p=0.048), anti-MDA5 (at 15UA: OR=4.36; 95% CI [1.19-15.99], p=0.048), anti-Mi2 (at 5UA: OR =3.24; 95% CI [1.01-10.46], p=0.026), anti-Jo1 (at 5UA: OR= 12.20; 95% CI [2.78-53.52], p&lt;0.0001). All positive predictive values were improved by using a clinical threshold although some of did not reach significance due to their infrequency. Conclusion In this retrospective work, despite missing data, the clinical phenotypes of myositis patients and their distribution according to aAbs were comparable to those in the literature. Our study confirms the lack of specificity of D-tek immunodots for IIM-specific aAbs and allows establishing new thresholds improving their performance. Our results should encourage medical biologists to establish local rules of interpretation and reinforce the interest of the discussion between Clinicians and Biologists around the interpretation of these immunodots. Figure 1. A: Odd Ratio (OR) evaluated with the manufacturer threshold ≥ 5 and B: OR using Youden’s index (clinical threshold). Disclosure of Interests None declared